The present invention relates to a method and apparatus for drilling in earthen formations for the production of gas, oil, and water. The system is also useful in mining operations and anywhere it is necessary to drill a hole of a particular diameter into the earth. In particular, the present invention relates to a method and apparatus for fluid jet-assisted mechanical drilling or mechanically-assisted fluid jet drilling. Although the invention is described herein in connection with gas and oil well drilling, the principles and concepts disclosed apply equally to other forms of drilling.
In oil and gas well drilling, the cost of equipment and labor is extremely high. In order to minimize the cost of this phase of oil and gas production, it is desirable to drill the holes through earthen formations, as rapidly as possible commensurate with good drilling practices. In drilling earthen formation holes, particularly in harder formations (which are more difficult to drill) and as the depth of the hole increases, there are a number of operating problems that tend to make the cost of such holes more expensive. Also, there are a number of tradeoffs and drilling factors which must be considered in order to maximize the rate of penetration of the drill bit and minimize the cost.
The primary sources of drilling forces which affect the rate of penetration during drilling are: (1) the torque provided by the rotation of the drill bit as it bores its way through the earthen formation, (2) the weight, supplied by that portion of the drill assembly known as the drill collar, acting on the drill as it presses against the formation, and (3) the pressure of the drilling fluid which is delivered to the drill bit through the drill string.
As the depth of the well increases, the drilling forces available to the drill bit as a result of the rotation of the drill bit and the pressure of the drilling mud are reduced because of transmission losses between the drilling rig and the bit. Furthermore, as the hole gets deeper, the earthen formations become more difficult to drill. Therefore, the rate of penetration decreases.
To maintain the rate of penetration the weight acting on the drill bit and its torque can be increased. However, where the weight on the drill bit is increased, the drill bit wears out much faster. It then becomes necessary to replace the drill bit more frequently. This is also a very undesirable trade-off since the entire drill string must be removed from the hole in order to replace the drill bit. For holes of 10,000 feet or more, replacing the bit often takes one or more days and is very costly.
Placing additional weight on the drill bit is also an undesirable trade-off because increased weight causes the bit to drill in unwanted directions (directional instability), which may cause expensive operating problems.
High pressure fluid jets provide a means of increasing the rate of penetration by increasing power levels at the bit without increasing directional control requirements. There are methods in current practice that use fluid jets to increase drilling rates. These methods involve increasing the fluid pressure of the conventional drilling mud stream from 2000 pounds per square inch (psi) to approximately 4000 psi. The added pressure is used to increase the velocity of the fluid leaving the nozzles. However, this is done only to assist in the removal of the cuttings, not to penetrate the rock. This method is commonly known as jet drilling and generally results in increased rates of penetration of about 30% to 50% over conventional approaches.
Experimental approaches have investigated higher pressure fluid jets as a means to assist the drilling process by actually cutting the rock with the jet. In one program in which target pressures of 15,000 psi were attempted, the rates of penetration increased by factors of 2 to 4 but apparently the system held together only for a short time duration. The system was designed to increase the entire mud stream pressures from pump through jet nozzles. This required surface power sources of around 5000 horsepower (hp) at mud flow rates of about 400 gallons per minute (gpm). Numerous operating problems ensued because of elevated pressures. Both pump and transmission systems (drilling assembly) failed in a relatively short time. The approach was proven to be technically effective, but not practical.